## Main-Program and Related Settings

### Introduction

The main program is up to the user to write. However, sample main programs are provided. In one such class of programs, key settings of the run are read in from a "cards file". For experimental collaborations this is actually the most common way to run a program like PYTHIA. The commands in such a file may be of two types
(a) instructions directly to `Pythia`, like which processes to generate, and
(b) instructions to the main program for what it should do, like how many events to generate, and how many events should be listed.
In principle these two kinds could be kept completely separate. However, to make life simpler, a few useful main-program settings are defined on this page, so that they are recognized by the `Settings` machinery. They can thus be put among the other cards without distinction. It is up to you to decide which ones, if any, you actually want to use when you write your main program.

To further reduce the necessary amount of main-program code, some of the tasks that you can steer from your main program can also be done internally. This in particular relates to some information printing. To give an example, `pythia.event.list()` can be inserted to print an event, i.e. all the particles belonging to it. Given the length of these listings one would list only a few events at the beginning of the run, to get some feeling for the character of events. This could be achieved e.g. with a main-program statement
`if (iEvent < 3) pythia.event.list()`
to list the first three events in a loop over `iEvent`, after `pythia.next()` has been used to generate the next event. Alternatively a `Next:numberShowEvent = 3` setting, e.g. in a command file, would achieve the same, by an internal call at the end of `pythia.next()`.

The settings names on this page thus fall into four main groups

- `Init:...` denote actions that automatically may be taken during the `pythia.init()` call.
- `Next:...` denote actions that automatically may be taken during the `pythia.next()` call.
- `Stat:...` denote actions that automatically may be taken during the `pythia.stat()` call.
- `Main:...` denote actions that you yourself have the freedom to make use of in your main program.

### Initialization settings

`flag` Init:showProcesses (`default = on`)
Print a list of all processes that will be simulated, with their estimated cross section maxima, as used for the subsequent Monte Carlo selection. Also print corresponding Les Houches initialization data, where relevant.

`flag` Init:showMultipartonInteractions (`default = on`)
Print initialization information for the multiparton interactions machinery.

`flag` Init:showChangedSettings (`default = on`)
Print a list of the changed flag/mode/parameter/word settings.

`flag` Init:showAllSettings (`default = off`)
Print a list of all flag/mode/parameter/word settings. Warning: this will be a long list.

`flag` Init:showChangedParticleData (`default = on`)
Print a list of particle and decay data for those particles that were changed (one way or another).

`flag` Init:showChangedResonanceData (`default = off`)
In the previous listing also include the resonances that are initialized at the beginning of a run and thus get new particle data, even if these may well agree with the default ones. Warning: this will be a rather long list.

`flag` Init:showAllParticleData (`default = off`)
Print a list of all particle and decay data. Warning: this will be a long list.

`mode` Init:showOneParticleData (`default = 0`; `minimum = 0`)
Print particle and decay data for the particle with this particular identity code. Default means that no particle is printed.

### Event-generation settings

`mode` Next:numberCount (`default = 1000`; `minimum = 0`)
Print a line telling how many events have been generated so far, once every `numberCount` events. If set zero then no lines are ever printed.
In `include/Pythia8Plugins/ProgressLog.h` an alternative method is implemented that intermittently prints out run progress information, reports on CPU usage and estimates when the run will end. It is used in the `main111.cc` example.

`mode` Next:numberShowLHA (`default = 1`; `minimum = 0`)
The number of events to list the Les Houches input information for, where relevant.

`mode` Next:numberShowInfo (`default = 1`; `minimum = 0`)
The number of events to list the `Info` information for, where relevant.

`mode` Next:numberShowProcess (`default = 1`; `minimum = 0`)
The number of events to list the `process` record for, where relevant.

`mode` Next:numberShowEvent (`default = 1`; `minimum = 0`)
The number of events to list the `event` record for, where relevant.

`flag` Next:showScaleAndVertex (`default = off`)
In addition to the normal information in the listing of the `process` and `event` records, a second line per particle provides information on the production scale, particle polarization and production vertex.

`flag` Next:showMothersAndDaughters (`default = off`)
In addition to the normal information in the listing of the `process` and `event` records, further lines list all the mothers and daughters of each particle.

### Statistics

`flag` Stat:showProcessLevel (`default = on`)
Print the available statistics on number of generated events and cross sections, where relevant.

`flag` Stat:showPartonLevel (`default = off`)
Print the available statistics on number and types of multiparton interactions, where relevant.

`flag` Stat:showErrors (`default = on`)
Print the available statistics on number and types of aborts, errors and warnings.

`flag` Stat:reset (`default = off`)
Reset the statistics of the above three kinds. The default is that all stored statistics information is unaffected by the `pythia.stat()` call. Counters are automatically reset in each new `pythia.init()` call, however, so the only time the reset option makes a difference is if `stat()` is called several times in a (sub)run.

### Main-program settings

The settings in this section must be under the control of the user, i.e. there are no internal equivalents. The first one is especially important and would be a standard feature of any separate command file.

`mode` Main:numberOfEvents (`default = 1000`; `minimum = 0`)
The number of events to be generated.

`mode` Main:numberOfTriedEvents (`default = 0`; `minimum = 0`)
The number of events to be tried during generation. Any number smaller than one means that the setting will be ignored.

`mode` Main:numberOfSelectedEvents (`default = 0`; `minimum = 0`)
The number of events to be selected during generation. Any number smaller than one means that the setting will be ignored.

`mode` Main:numberOfAcceptedEvents (`default = 0`; `minimum = 0`)
The number of events to be accepted during generation. Any number smaller than one means that the setting will be ignored.

`mode` Main:timesAllowErrors (`default = 10`)
Allow this many times that `pythia.next()` returns false, i.e. that an event is flawed, before aborting the run.

The `Main:...` options works like this. Once you have used the `pythia.readFile(fileName)` method to read in the cards file, where the values have been set, you can interrogate the `Settings` database to make the values available in your main program. A slight complication is that you need to use a different `Settings` method for each of the four possible return types that you want to extract. To save some typing the same method names are found directly in the `Pythia` class, and these just send on to the `Settings` ones to do the job, e.g.
`int nEvent = pythia.mode("Main:numberOfEvents");`

The area of subruns is covered separately below. A few spares are also defined after that, for unforeseen applications.

### Subruns

You can use subruns to carry out several tasks in the same run. In that case you will need repeated instances of the first setting below in your command file, and could additionally use the second and third as well.

`mode` Main:subrun (`default = -999`; `minimum = 0`)
The number of the current subrun, a non-negative integer, put as first line in a section of lines to be read for this particular subrun.

`flag` Main:LHEFskipInit (`default = off`)
If you read several Les Houches Event Files that you want to see considered as one single combined event sample you can set this flag `on` after the first subrun to skip (most of) the (re-)initialization step.

`mode` Main:numberOfSubruns (`default = 0`)
The number of subruns you intend to use in your current run. Unlike the two settings above, `Pythia` itself will not interpret this number, but you could e.g. have a loop in your main program to loop over subruns from 0 through `numberOfSubruns - 1`.

### Spares

For currently unforeseen purposes, a few dummy settings are made available here. The user can set the desired value in a "cards file" and then use that value in the main program as desired.

`flag` Main:spareFlag1 (`default = off`)

`flag` Main:spareFlag2 (`default = off`)

`flag` Main:spareFlag3 (`default = off`)

`mode` Main:spareMode1 (`default = 0`)

`mode` Main:spareMode2 (`default = 0`)

`mode` Main:spareMode3 (`default = 0`)

`parm` Main:spareParm1 (`default = 0.`)

`parm` Main:spareParm2 (`default = 0.`)

`parm` Main:spareParm3 (`default = 0.`)

`word` Main:spareWord1 (`default = void`)

`word` Main:spareWord2 (`default = void`)

`word` Main:spareWord3 (`default = void`)